Something has been in the back of my mind for awhile now.... some fellow boaters told me it's nothing to worry about.... but why would my engine temperatures be pointing up (normal) until I back off the throttles to an idle coming off plane, then the needles drop off to nothing, sometimes for a few seconds, sometimes more?

454 Mag engines

Any thoughts?

Could be a couple of reasons. One that comes to mind is that your drive could be sucking some air when running which is less effective at cooling than water. When you pull off plane, the drive is deeper and in clear water. The temps drop off because the cold water is suddenly cooling the engine faster than the termostat can close. Over time thermal shock could be a problem. The worst thing is that the temp sensor may actually be reading low as it might not be submurged. The sensor will read steam, but it's not as effective as water. Take a moment to confirm the engine temps are not actually higher than what the gauge is reading. If they truly are OK, then don't worry about it.

Wonder if the depth of the water may change it too? the water 3 ft down is certainly cooler than water 1 ft from the top.

Thanks, good input.

It may just be time to rebuild my water pumps and change my thermostats.

A couple other symptoms I should have included in the first post.... after a few seconds if the temp doesn't come back up, if I rev the engine a couple times the gauge will come back up to normal. (sometimes, one engine comes back shortly and one doesn't) Also, I'll notice a bit of steam coming out the exhaust.
I'm starting to wonder if I'm getting an air pocket to the temp senser, and revving the engine 'burbs' it out. I just couldn't understand where the air might be coming from, but could be cavitation to the pick-up I suppose.

Seems to happen if I come off plane too quickly, going back to my marina... thru a 30 mph zone and coming off plane very gradually at the entrance, temps may go down a little but they don't seem to drop off and stay there.